This 'Dark Night' presents significant risk (bricking) that requires careful adherence to the installation process. The NVRAM issue has been resolved, however, knowing what builds can be used must be understood well.

Pressing the Reset button at powerup will put the N66U into recovery mode.

Press and hold the reset button while plugging in the power, and keep pressing the reset button for about ten seconds until the power led is blinking slowly, then release the reset button. The router is now in recovery mode: go to 192.168.1.1.

Press and hold the WPS button while plugging in the power, and keep pressing the WPS button for about ten seconds, then release the WPS button. All settings have now been wiped, and default settings will be used.

IMPORTANT: DO NOT USE K2.6 builds if using CFE v1.0.1.3 (64K nvram) or NEWER!!!
At launch, the RT-N66U was limited to 32k nvram. However, CFE (bootloader) version 1.0.1.3 added 64k nvram support around April 2013, but was not compatible with DD-WRT firmware at that time. For details, see these forum links to historical nvram and 64K implemention discussions: Fractal comments and "RT-N66 CFE Thread"

This is only potentially relevant if using builds from 2015 and earlier.

If you are already on a custom 64K Build like Fractal or Kong with CFE 1.0.1.3, you need to flash 26138 mega nv64k. Since K2.6 Builds have an embedded check, you need to use those special nvXXk trailed images. On K3.X this check has been removed and you can simply upgrade from a previous K3.X.

25-May-2013: With the new experimental K3.X Broadcom builds, the CFE version is irrelevant. DD-WRT detects it automatically to extend 32KB NVRAM to 64KB. Which Image you have to flash, depends on your previous Image (Stock, Merlin, DD-WRT K2.6).

9-Apr-2013: It seems more recently manufactured units come pre-loaded with the 64K v1.0.1.3 CFE and so are not compatible with standard DD-WRT builds. Flashing a K2.6 DD-WRT build on these devices will result in a brick.

26-Jan-2013: Current versions of DD-WRT can quickly run out of 32k nvram.

Spacers (~1/4" to 10mm) can be installed to elevate the case top, which allows easier access to the MicroSD card, and also improve cooling. A piece of dark foam and hole punch can be used to retain the top cover LED lighting.